The present invention relates to an active matrix liquid crystal display device. More particularly, the present invention relates to an active matrix liquid crystal display device which is configured in accordance with the IPS (In-Plane Switching) type (also called the lateral electric field type).
In recent years, active matrix liquid crystal display devices employing active elements, represented by thin film transistors (TFTs), have been increasingly used as monitors for personal computers, workstations, and so on since they consume less power and have smaller sizes than CRT display devices while providing a high image quality equivalent to that of the CRT display devices.
One form of a liquid crystal display device suitable for monitor applications is an IPS-type active matrix liquid crystal display device. The liquid crystal display device of this type includes scanning wires, signal wires, common wires, and pairs of interdigitally formed electrodes (pixel electrode and opposing electrode) arranged on one of the two substrates, where a voltage is applied across the electrodes to drive liquid crystal. An electric field applied to the liquid crystal is substantially parallel to the surfaces of the substrates. The liquid crystal display device of IPS-type has a wider viewing angle than conventional liquid crystal display devices. This characteristic of the IPS-type liquid crystal display device makes itself more suitable for applications in direct-view type monitors.
FIG. 4 illustrates in plan view the structure of one pixel section in a conventional IPS-type active matrix liquid crystal display device as mentioned. FIG. 5 illustrates the structure of FIG. 4 in cross-sectional view taken along a line B-Bxe2x80x2 in FIG. 4.
Referring first to FIG. 4, the pixel section includes scanning wires 101 formed of Cr; a semiconductor layer 102 formed of amorphus silicon; a signal wire 103 formed of Cr; a pixel electrode 106 formed of Cr; opposing electrodes 107 and 107xe2x80x2 formed of Cr; a common wire 401 formed of Cr; and a black matrix 402.
The liquid crystal display device having the structure as illustrated includes a gap between the opposing electrode 107 and the signal wire 103. An effective electric field to the pixel for display cannot be applied through this gap. In addition, this gap must be shielded because light is likely to leak from the gap due to a continuously changing voltage on the signal wire 103.
Moreover, a gap between the scanning wire 101 and the common wire 401 must be shielded since light is likely to leak from the gap due to a direct current voltage applied at all times to these wires. Furthermore, gaps between the scanning wire 101 and ends of the opposing electrodes 107, 107xe2x80x2 must be shielded for the same reason. In addition, a thin film transistor (TFT) must be shielded over for preventing the TFT from malfunctioning due to a current possibly caused by leaked light. Thus, the liquid crystal display device has a low aperture ratio because the pixel section must be shielded by the black matrix 402. Further, the existence of the common electrode 401 also contributes to the low aperture ratio of the liquid crystal display device.
As illustrated in FIG. 5, a section in which a TFT 216 is arranged presents an abruptly narrowing gap between a TFT substrate 214 and an opposing substrate 215. This is because:
1) a thicker passivation layer 205 for the purpose of planarizing the TFT substrate 214 cannot be employed because of a resulting increase in a liquid crystal driving voltage; and
2) the black matrix 204 provided on the opposing substrate 215 in face of the TFT section cannot be omitted since the black matrix 204 is indispensable for preventing the TFT from malfunctioning due to a current generated by leaked light.
In the exemplary liquid crystal display device, a spacer bead (hereinafter simply called the xe2x80x9cbeadxe2x80x9d) 211 positioned on the TFT section serves to define a cell gap. However, since the area of the TFT section is merely on the order of {fraction (1/100)} as much as an entire pixel, the beads 211 arranged on the TFT sections account for only {fraction (1/100)} or less of the entire area of the substrates. For this reason, in substrate regions without the TFT sections and accordingly not supported by the spacer beads 211 arranged therebetween, the cell gap suffers from non-uniformity which in turn causes a non-uniform display luminance.
Nevertheless, if an increased amount of beads 211 is dispersed in an attempt to make the cell gap more uniform, the number of beads increases not only on the TFT sections but also at openings. Thus, while the cell gap can be made more uniform, more light leaks near beads positioned at openings, resulting in a lower contract.
In the cross-sectional view of FIG. 5, the structure further comprises a glass substrate 201; a gate insulating layer 202; a contact layer 204; an alignment film 206; a glass substrate 207; a color filter layer 208; a protection film 209 which also serves as a planarizing film; an alignment film 210; a liquid crystal layer 212; and polarizing plates 213, 217.
When compared with a conventional TN-type liquid crystal display device, the IPS-type liquid crystal display device has the following two problems to be solved.
First, higher power consumption is required. This is because a low aperture ratio of the IPS-type liquid crystal display device requires higher power consumption to drive back light for providing the luminance equivalent to that of the conventional TN-type liquid crystal display device.
The low aperture ratio of the IPS-type liquid crystal display device is mainly caused by the following facts:
1) interdigital electrodes do not transmit light; and
2) a black matrix (hereinafter abbreviated as xe2x80x9cBMxe2x80x9d) arranged on an opposing substrate partially blocks openings from receiving light.
Regions blocked by the BM are edge portions of scanning wires and signal wires, and TFT sections.
The edge portions of scanning wires and signal wires are shielded because light may leak from these portions. The TFT sections are shielded for preventing TFTs from malfunctioning due to a current generated by leaked light.
The BM has an area which is typically set larger than the sum of possible light leaking regions and TFT regions for taking into account an allowance of the alignment of a substrate which has formed thereon the possible light leaking regions and the TFT regions to be shielded (hereinafter called the xe2x80x9cTFT substratexe2x80x9d), to another substrate on which the BM is formed (hereinafter called the xe2x80x9copposing substratexe2x80x9d).
This wide area of the BM contributes to an additional reduction in the aperture ratio. The aperture ratio must be increased in order to reduce power consumption.
Another problem is that the IPS-type liquid crystal display device suffers from a low uniformity of display luminance. This is because in the IPS-type liquid crystal display device, a threshold voltage for the luminance characteristic is inversely proportional to the thickness of a liquid crystal layer sandwiched between the pair of substrates (cell gap), so that non-uniformity of the cell gap, if any, would appear in a display as corresponding non-uniformity of luminance.
In the TN-type liquid crystal display device, on the other hand, a threshold voltage does not depend on a cell gap, so that the uniformity of display luminance is relatively high. In order to improve the uniformity of display luminance in the IPS-type liquid crystal display device, the uniformity of the cell gap between the two substrates must be ensured more strictly than the TN-type liquid crystal display device. However, an attempt to make the cell gap more uniform using currently available methods would result in a lower contrast which constitutes a further problem. In the following, the cause of the second problem will be explained in detail.
The non-uniform cell gap and the reduced contrast caused by an attempt to make the cell gap more uniform attribute to the ruggedness of the opposing surfaces of the two substrates and the beads used to form the cell gap.
When the cell gap is formed by sandwiching beads between such rugged surfaces of two substrates, the cell gap is defined by those beads that are sandwiched between regions in which the spacing between the substrates is the narrowest. In the active matrix liquid crystal display device, it is the TFT section that has the narrowest spacing between the substrates. This is because the TFT section has such a structure that the most protruding TFT section on one substrate faces the most protruding section on the other substrate in which a black matrix for shielding the TFT overlaps a color filter, so that the spacing between the substrates is the narrowest in this region.
The TFT section has an area approximately {fraction (1/100)} a pixel area. Thus, when beads are dispersed over the TFT substrate, beads would be sandwiched between the TFT sections of the TFT substrate and corresponding sections of the opposing substrate with a possibility of {fraction (1/100)} or lower, in additional consideration of the fact that the beads are carried on the TFT sections with more difficulties than on other flat regions. For example, when 100 beads are dispersed, 99 beads will be positioned on pixel sections other than the TFT sections, and thus will not contribute to supporting the substrates. Regions in which the substrates are not supported by beads are more likely to suffer from a non-uniform cell gap which would give rise to the non-uniformity of display luminance in the IPS-type liquid crystal display device.
To improve the uniformity of display luminance, the cell gap must be made more uniform. When a larger amount of beads is dispersed to improve the uniformity of the cell gap, an increased number of beads may be positioned in the TFT sections, whereas the number of beads positioned in openings is also increased in proportion.
Generally, near beads, light is more likely to leak due to defective alignment of liquid crystal. For this reason, while the increase in the number of beads might provide a more uniform cell gap, the contrast would be degraded due to light leaking near the beads positioned in openings. Thus, when the cell gap is made more uniform by dispersing an increased amount of beads, the contrast is degraded. To reduce the amount of dispersed beads required for a higher uniformity of the cell gap, the ruggedness on the opposing surfaces of a pair of substrates must be reduced to planarize the surfaces.
In summary, the IPS-type active matrix liquid crystal display device has the following problems:
1) high power consumption; and
2) low uniformity of display luminance.
To overcome these problems,
1) the aperture ratio must be increased to reduce power consumption for driving the back light; and
2) the cell gap must be made more uniform while avoiding a degraded contrast.
It is an object of the present invention to provide an IPS-type active matrix liquid crystal display device which exhibits a high contrast in a displayed image.
It is another object of the present invention to provide an IPS-type active matrix liquid crystal display device which is capable of eliminating a light shielding film required in conventional IPS-type active matrix liquid crystal display devices.
As a first structure to solve the above problems, in an IPS-type active matrix liquid crystal display device, a portion of an opposing electrode for driving a liquid crystal for display together with a pixel electrode is formed over a signal wire through an insulating film. Then, a region having the signal wire formed therein, viewed from a direction perpendicular to the surface of a substrate, is included entirely within a region having the opposing electrode formed therein and a region having a scanning wire formed therein, without protruding therefrom. Alternatively, the portion of the opposing electrode may be formed over an active element through an insulating film.
A first feature of this structure lies in a common wire eliminated configuration (common-less configuration), as described in JP-A-8-62578, in which the scanning wire functions also as a common wire while omitting the common wire which is included in conventional IPS-type active matrix liquid crystal display devices. By employing the structure based on the common wire eliminated configuration, it is possible to eliminate a BM arranged on the opposing substrate in conventional liquid crystal display devices. This results in improving the aperture ratio as well as the planarity of the opposing substrates of the liquid crystal display device.
The followings are the reasons why the present invention can eliminate BM which is required in conventional liquid crystal display devices for shielding the active element and possible light leaking regions in edge portions of the scanning wire and the signal wire:
1) the opposing electrode arranged over the active element functions as a light shielding layer;
2) since the signal wire is completely covered with the opposing electrode and the scan electrode, light leaking from edge portions of the signal wire is eliminated; and
3) Since the common wire eliminated configuration is inherently free of light leaking from edge portions of the scanning wires, light shielding is not required.
Within the above reasons, since the signal wire is completely covered with the opposing electrode and the scanning wire as set forth in 2), the structure of the present invention is realized by forming the opposing electrode such that its end portion is placed over the corresponding scanning wire.
The light leaking from edge portions of the signal wire can be completely eliminated by employing the structure as described above because the present invention employs the common wire eliminated configuration.
As described in JP-A-8-62578, in the common wire eliminated configuration, equal voltages are applied across opposing electrodes and corresponding scanning wires during most of a liquid crystal display operating period, so that light will not leak even if an end portion of the opposing electrode is placed over the scanning wire.
On the other hand, in a conventional structure having a common wire, a direct current is applied between an opposing electrode and a corresponding scanning wire during most of a liquid crystal display operating period, so that the placement of an end portion of the opposing electrode in close proximity to the corresponding scanning wire would cause light to leak in a region between the end portion of the opposing electrode and the corresponding scanning wire due to the applied direct current voltage, although such placement reduces regions of edge portions of the signal wire from which light may leak.
Thus, even if the structure of the present invention, having an end portion of an opposing electrode placed over a corresponding scanning wire, were applied to the conventional structure, it would not be possible to completely eliminate light leaking from edge portions of the signal wire. In other words, the present invention can be effective exclusively in the common-less configuration which does not have a common electrode.
Also, in the foregoing structure which involves a plurality of opposing electrodes in contact with a liquid crystal layer through an alignment film, the plurality of opposing electrodes are preferably made of an electro-chemically stable material such as niobium (Nb) or niobium nitride in order to prevent possible failures caused by electro-chemical reactions between a liquid crystal composition and the opposing electrodes.
Since a cell gap is defined by using columnar spacers of a uniform height formed on the TFT substrate, instead of beads previously used for the formation of the cell gap, and sandwiching the spacers between the pair of substrates, the uniformity of the cell gap is improved. Preferably, the columnar spacers are positioned at regular intervals corresponding to periodic placement of pixel electrodes.
Particularly, when the spacers are positioned over the active elements through the opposing electrodes, the columnar spacers can have a minimum height because the spacing between the substrates is the narrowest in regions in which the active elements are formed.
It is therefore possible to reduce the amount of materials required for the columnar spacers and hence a time required to form the columnar spacers.
In addition, since the columnar spacer is arranged over the active element through the opposing electrode, the potential over the active element is maintained at the potential of the opposing electrode and is free from the influence of the columnar spacer, so that the placement of the columnar spacer over the active element will never cause the active element to malfunction.
The opposing electrodes may be made of an electrically conductive oxide such as indium tin oxide or the like which is electrochemically stable. However, since this material is transparent, light shielding is required for the active elements or TFTs. In this event, when columnar spacers having a light shielding property are arranged over the active elements through the opposing electrodes, it is possible to simultaneously achieve a uniform cell gap and light shielded TFTs. Alternatively, a small black matrix may be formed over the TFT for light shielding.
When a TFT is used as the active element, the opposing electrode arranged over the TFT through an insulating layer has a function of a gate electrode (a so-called double gate TFT structure), thus providing an additional effect of an increased mobility and enhanced switching performance for the TFT.